The present invention relates to a solid electrolyte with ionic conductivity, which. can be used in batteries, capacitors, electrochromic devices (ECD), sensors, etc.
Solid electrolytes with ionic conductivity have heretofore been proposed, which are obtained, for example, by mixing an acryloyl-modified polymer compound having an alkylene oxide polymer chain and an electrolytic salt optionally along with a solvent followed by crosslinking the mixture by heating it or exposing it to light or electronic rays.
For example, known are a solid polyelectrolyte comprising a combination of a tri-functional polymer having a terminal acryloyl-modified alkylene oxide polymer chain, a low-molecular alkylene oxide copolymer, polyvinyl chloride and an electrolytic salt (see Japanese Patent Application Laid-Open (JP-A) Hei-3-177409), a solid electrolyte comprising a combination of the same terminal acryloyl-modified alkylene oxide copolymer as above, an inorganic ionic salt and an organic solvent such as propylene carbonate (see JP-A Sho-63-94501), and a solid electrolyte comprising a combination of a bi-functional and/or mono-functional polymer compound having a terminal acryloyl-modified alkylene oxide polymer chain, and an electrolytic salt (see JP-A Hei-5-178948).
However, those conventional solid electrolytes that are obtained by exposing them to active radiations and/or heating them are problematic in that the storage stability of the cured products is poor because of the non-crosslinked monomers still remaining in them.
The present invention has been made in consideration of the problems in the prior art, and the subject matter of the invention is to provide a solid electrolyte, which, when cured under the same condition as that for the conventional solid electrolytes noted above, contains a reduced amount of non-crosslinked monomers, which has an excellent film-forming ability as being able to be cured rapidly, and of which the electroconductivity is comparable to that of conventional electrolytic solutions.
The solid electrolyte of the present invention is prepared by crosslinking a composition that consists essentially of a polymer compound, a solvent and an electrolytic salt through exposure to active radiations and/or under heat, and is characterized in that the polymer compound has four functional polymer chains of a general formula (I): 
wherein R1 and R2 each represent a hydrogen atom or a lower alkyl group;
R3 represents a hydrogen atom or a methyl group;
m and n each represent 0 or an integer of 1 or more, and m+nxe2x89xa735 in one polymer chain; and
wherein R1 R2, R3, m and n each may be the same or different in the four functional polymer chains.
The polymer compound is hereinunder referred to as xe2x80x9ctetra-functional, terminal acryloyl-modified alkylene oxide polymerxe2x80x9d.
In the solid electrolyte, the polymer compound may have a structure of a general formula (II): 
wherein R represents a residue of a starting substance, R1, R2, R3, m and n have the same meanings as in formula (I). Those four R1""s, R2""s, R3""s, m""s and n""s each may be the same or different in one molecule of the polymer compound.
In each of formulas (I) and (II), (a) if m and n is each an integer of 1 or more, then at least one of R1 and R2 is a lower alkyl group, (b) if m is 0, then R2 is a lower alkyl group, and (c) if n is 0, then R1 is a lower alkyl group.
In the solid electrolyte of the invention, the solvent is preferably at least one selected from the group consisting of cyclic esters, cyclic carbonates, cyclic ethers, nitrites, linear ethers, linear carboxylates, linear carbonates, sulfolane, sulfolane derivatives, dimethylsulfoxide, N,N-dimethylformamide, and N-methyloxazolidinone.
Of that type, the amount of the solvent in the solid electrolyte is preferably from 220 to 1,900% by weight relative to the polymer compound therein.
To obtain the tetra-functional, terminal acryloyl-modified alkylene oxide polymer for use in the invention, for example, an active hydrogen compound, such as diglycerin or pentaerythritol, is used as the starting substance, and an alkylene oxide, which is mentioned hereinunder, is added thereto, and is then esterified with an unsaturated organic acid such as acrylic acid or methacrylic acid, or is then reacted with an acid chloride such as acrylic acid chloride or methacrylic acid chloride through dehydrochlorination.
Specific examples of the polymer are mentioned below with reference to R1, R2, R3, m and n in formula (II). 
R1: xe2x80x94H, xe2x80x94CH3, xe2x80x94C2H5, R2: xe2x80x94H, xe2x80x94CH3, xe2x80x94C2H5 R3: xe2x80x94H, xe2x80x94CH3, m: 0xcx9c130, n: 0xcx9c130.
The active hydrogen compound to be used as the starting substance may have four functional groups, of which the type is not specifically defined. However, preferred are diglycerin and pentaerythritol, as having high reactivity with alkylene oxides. Apart from these, also employable are methyl glucoside, ethylene diamine and aromatic diamines.
The alkylene oxides to be used in producing those tetra-functional alkylene oxide polymers include, for example, ethylene oxide, propylene oxide, 1,2-epoxyhexane, 1,2-epoxyoctane, etc. Preferred are ethylene oxide, propylene oxide, and butylene oxide. The number of the monomers constituting each functional polymer chain, or that is, each polyalkylene oxide chain in the tetra-functional alkylene oxide polymer must be not smaller than 35, preferably from 35 to 150, more preferably from 40 to 120.
If the number of the monomers in each unit chain is smaller than 35, such is problematic in that the polymer is difficult to crosslink in a solvent of being not smaller than 220% by weight relative to the polymer and that the solvent used greatly bleeds out onto the surface of the crosslinked product. Where two different monomers are used to produce the polymer, the sequence of the monomer units in the polymer is not specifically defined, and the polymer may be any of block or random copolymers.
Any and every solvent well miscible with the polymer is employable to give the solid electrolyte of the invention. However, preferred are one or more selected from the group consisting of cyclic esters, cyclic carbonates, cyclic ethers, nitrites, linear ethers, linear carboxylates, linear carbonates, sulfolane, sulfolane derivatives, dimethylsulfoxide, N,N-dimethylformamide, and N-methyloxazolidinone, as they can dissolve ionic compounds and have good conductivity. Of those, especially preferred are cyclic esters and cyclic carbonates.
The ratio of the solvent to the polymer is generally from 220 to 1,900% by weight, but preferably from 220 to 1,200% by weight, more preferably from 230 to 1,000% by weight. If the amount of the solvent is smaller than 220% by weight, the conductivity of the solid electrolyte obtained will be low. On the other hand, if the amount is larger than 1,900% by weight, the mechanical strength of the solid electrolyte will be greatly lowered.
The solid electrolytic salt for use in the invention is at least one selected from the group consisting of lithium fluoride, lithium chloride, lithium bromide, lithium iodide, lithium nitrate, lithium thiocyanate, lithium perchlorate, lithium trifluoromethanesulfonate, lithium tetraborofluoride, bistrifluoromethylsulfonylimide lithium, tristrifluoromethylsulfonylmethide lithium, sodium thiocyanate, sodium perchlorate, sodium trifluoromethanesulfonate, sodium tetraborofluoride, potassium thiocyanate, potassium perchlorate, potassium trifluoromethanesulfonate, potassium tetraborofluoride, magnesium thiocyanate, magnesium perchlorate and magnesium trifluoromethanesulfonate. The ratio of the electrolytic salt to the solvent is generally from 0.2 to 3.0 mols/liter, but preferably from 0.5 to 2.0 mols/liter.
As the means of crosslinking the polymer-containing composition to obtain the solid electrolyte of the invention, employable are active radiations such as UV rays, visible rays and electron rays. Apart from those, heating is also effective.
If desired, a photopolymerization initiator such as trimethylsilylbenzophenone, benzoin, 2-methylbenzoin, 4-methoxybenzophenone, benzoin methyl ether-anthraquinone or benzyldimethyl ketal, or a polymerization initiator such as benzoyl peroxide, methyl ethyl ketone peroxide or xcex1,xcex1xe2x80x2-azobisisobutyronitrile may be added to the composition being crosslinked.
The method of producing the solid electrolyte of the invention is not specifically defined. For example, the polymer is mixed with a solvent that contains an electrolytic salt having been dissolved therein to give a uniform mixture; or the polymer is uniformly mixed with a solvent and an electrolytic salt. The resulting solution is applied onto a substrate, for example, through bar-coating, spin-coating or screen-coating to form thereon a uniform layer, which is then crosslinked according to the means mentioned above. Thus is easily obtained the solid electrolyte of the invention.